Morphine suppresses MHC-II expression on circulating B lymphocytes via activation of the HPA.

Morphine has been shown to alter gene expression of the major histocompatibility complex, class II (MHC-II) in circulating rat immunocytes. Here, we demonstrate that a single morphine injection (10 mg/kg) reduces basal MHC-II protein expression on circulating B lymphocytes by 33%, while also impairing the ability of B lymphocytes to increase MHC-II upon interleukin-4 induction. As these data implicate opioids in the regulation of antigen presentation, studies were undertaken to examine the potential mechanisms through which morphine exerts this suppressive effect. Central injection studies utilized Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO), an opioid receptor agonist, which mimicked morphine's effect on MHC-II, while D-Phe-Cys_Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) pretreatment, prior to morphine, blocked the suppression of MHC-II. As central opioid receptor activation results in the activation of the hypothalamic-pituitary-adrenal axis, thereby, signaling increased circulating corticosterone levels, we examined whether MHC-II expression was suppressed after incubation with corticosterone at concentrations similar to those observed after morphine. Interestingly, corticosterone dramatically decreased basal MHC-II (88%) expression while completely preventing the induction of MHC-II. Additionally, MHC-II suppression was absent in morphine-treated adrenalectomized animals. Since prolonged morphine exposure has previously been shown to result in tolerance to both the steroidogenic and immunosuppressive effects of morphine, the effect of prolonged morphine exposure on MHC-II was also examined. Interestingly, MHC-II expression is no longer suppressed after chronic morphine, while morphine withdrawal results in both a renewed increase in circulating corticosterone levels and a renewed suppression of MHC-II in previously tolerant animals. Taken together, these data strongly implicate corticosterone in mediating the suppressive effects of morphine on circulating B-lymphocyte MHC-II expression.

Enhanced immune sensitivity to stress following chronic morphine exposure.

Chronic administration of escalating doses ofmorphine leads to neuroadaptive changes precipitating development of tolerance to many of the acute effects of morphine, such as analgesia, activation of the hypothalamic-pituitary-adrenal (HPA) axis and suppression of immune cell activities. Interestingly, morphine tolerance has also been shown to be accompanied by heightened immunosuppressive effects of restraint stress using a rodent model. These observations have led to the hypothesis that the altered neuronal state accompanying opioid tolerance may contribute to this enhanced immune sensitivity to stress. To further test this hypothesis using different stressors, Sprague-Dawley rats were treated chronically with morphine for at least 8 days and then challenged with either psychological (water stress) or systemic stressors [morphine withdrawal, lipopolysaccharide (10 mug/kg i.p. challenge)]. It was found that, independent of the type of stress employed, morphine-tolerant animals displayed significantly lower mitogen-stimulated blood lymphocyte responses when compared to the responses of similarly treated saline controls. To determine whether direct activation of central stress pathways may also lead to enhanced immune sensitivity, morphine-tolerant animals were centrally injected with IL-1beta (1 ng/mul i.c.v.), a cytokine that activates the HPA axis by central mechanisms. Similar to the other types of stress, this direct central challenge was also found to be more immunosuppressive in morphine-tolerant animals compared to controls. Collectively, these studies demonstrate that morphine-tolerant animals have an enhanced susceptibility to the debilitating effects of a variety of stressors on immune cell function, an effect that is likely due to the neuroadaptive changes that develop during chronic morphine exposure.

Immune cell activity during the initial stages of withdrawal from chronic exposure to cocaine or morphine.

The immunosuppression accompanying illicit drug use has been shown to contribute to a decreased resistance to a variety of pathogens; however, there is relatively little information on how long these effects persist following withdrawal from chronic drug exposure. To begin to address this question, Sprague-Dawley male rats were administered either cocaine (10 mg/kg, i.p., b.i.d.) for 7 days or morphine (escalating doses up to 40 mg/kg, s.c., b.i.d.) for a 10-day period. Control groups of animals received similar saline injections for equivalent time periods. Drug administration was abruptly discontinued and animals were sacrificed at 2, 24, 72 or 96 h following the last dose. At these time points, proliferation responses of peripheral blood T-lymphocytes stimulated by concanavalin A (Con A) and plasma levels of corticosterone were measured. Plasma corticosterone levels of cocaine- or morphine-treated animals were found to be significantly elevated 24 h following drug cessation as compared to saline animals. At this time, proliferation responses were significantly decreased and were further suppressed during cocaine and morphine withdrawal at 96 and 72 h, respectively. These results suggest that abrupt cessation of cocaine or morphine administration leads to activation of stress-related pathways that may contribute to an increased susceptibility of infection during the initial withdrawal phase.

Differences in cytokine gene expression profile between acute and secondary injury in adult rat spinal cord.

It is likely that the environment within the injured spinal cord influences the capacity of fetal spinal cord transplants to support axonal growth. We have recently demonstrated that fetal spinal cord transplants and neurotrophin administration support axonal regeneration after spinal cord transection, and that the distance and amount of axonal growth is greater when these treatments are delayed by several weeks after injury. In this study, we sought to determine whether differences in inflammatory mediators exist between the acutely injured spinal cord and the spinal cord after a second injury and re-section, which could provide a more favorable environment for the axonal re-growth. The results of this study show a more rapid induction of transforming growth factor (TGF) beta1 mRNA expression in the re-injured spinal cord than the acutely injured spinal cord and an attenuation of proinflammatory cytokine mRNA expression. Furthermore, there was a rapid recruitment of activated microglia/macrophages in the degenerating white matter rostral and caudal to the injury but fewer within the lesion site itself. These findings suggest that the augmentation of TGFbeta-1 gene expression and the attenuation of pro-inflammatory cytokine gene expression combined with an altered distribution of activated microglia/macrophages in the re-injured spinal cord might create a more favorable milieu for transplants and axonal regrowth as compared to the acutely injured spinal cord.

Antagonism of N-methyl-D-aspartate receptors reduces the vulnerability of the immune system to stress after chronic morphine.

It has been shown that morphine-tolerant animals have an altered immunological sensitivity to stress. Although the glutamatergic system has been implicated in the neuroadaptive process underlying this tolerant state, its potential role in development of the altered immunological sensitivity consequent to chronic morphine treatment is not known. To determine this, a morphine-tolerant state was induced by 10-day administration of an escalating dose of morphine from 10 to 40 mg/kg (s.c., b.i.d.), and lymphocyte proliferative response to a T-cell mitogen was measured. Morphine challenge (10 mg/kg s.c.) after days of treatment was gradually less immunosuppressive, and this tolerance progression was delayed by concurrent administration of the N-methyl-D-aspartate (NMDA) receptor antagonist (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) (0.1 mg/kg s.c.) with chronic morphine. The effect was independent of glucocorticoid level changes and was not a result of an acute interaction of the drugs or the prolonged presence of the antagonist alone. Subsequent to chronic treatment, animals were subjected to opioid withdrawal and water stress. Both stressors induced 50% immunosuppression in morphine-tolerant animals compared with saline-treated controls. Increased immunological sensitivity to these stressors was attenuated when MK-801 was administered with chronic morphine as demonstrated by an accelerated recovery rate and lack of immunosuppression from opioid withdrawal and water stress, respectively. Together, these findings provide the first evidence that the neuroadapted state of the immune response after chronic morphine can be modified by NMDA receptor antagonism, as illustrated by a temporal deceleration of the development of immunological tolerance during chronic treatment that is associated with an attenuation of the immunological vulnerability of morphine-tolerant animals to stress.

Stress-induced suppression of the immune system after withdrawal from chronic cocaine.

Recent evidence suggests that withdrawal from cocaine shares similarities to the stress response. Here, we examine whether withdrawal from chronic cocaine produces immune system alterations and whether the hypothalamic-pituitary-adrenal axis is involved. Sprague-Dawley male rats received cocaine (10 mg/kg i.p., b.i.d.) or saline, followed by 2 h, 1, 2, 4, 6, and 14 days of withdrawal. Proliferation responses of peripheral blood lymphocytes to concanavalin A were significantly suppressed at the 2-h, 1- and 2-day time points, and persisted for up to 6 days during withdrawal from chronic cocaine. Flow cytometric analysis revealed no significant differences in the immunophenotype of blood lymphocytic populations of T cells, B cells, or monocytes at 2 or 6 days of withdrawal from cocaine. Consistent with the suppression in cellular immunity observed in the in vitro response, the in vivo delayed-type hypersensitivity response was also significantly decreased in cocaine withdrawing animals. Plasma corticosterone levels were significantly elevated 2 and 24 h after cessation of cocaine but returned to basal values by 2 days of withdrawal. The suppressive effects of cocaine withdrawal were no longer observed in either adrenalectomized animals or those treated with the glucocorticoid receptor antagonist mifepristone (RU486), when administered during the first 2 days of withdrawal. These data argue that repeated exposure to cocaine followed by withdrawal leads to an activation of the neuroendocrine stress response, which alters cellular immunity during the initial withdrawal phase and may contribute to an increased susceptibility to infection.

Opioids, immunology, and host defenses of intravenous drug abusers.

Overall, it is apparent that opioids do affect host defense mechanisms. Heroin users present with an altered and functionally impaired immune system and have a higher prevalence of infectious diseases than do nonaddicts. Individuals exposed to opioid treatment for pain management during surgical procedures or maintained on oral methadone for treatment of drug addiction show either no effect or a suppressed immune system, depending on dosage and, in the case of methadone-maintained patients, duration of drug treatment. Confounding factors in these studies undermine definitive conclusions about the mechanisms by which opioids induce their immunomodulatory effects. Animal models have provided the means by which investigators can study the effects of opioids in a complex, biologic system that is easily manipulated and controlled. Findings from these studies have confirmed human data associating a pathogenic susceptibility with opioid use. Animal models have shown the complexity of this association. Interaction of the CNS, the autonomic nervous system, and the HPA axis is required for the varied effects of opioids on the immune system. By implication, exogenous opioids may be mimicking pathways by which endogenous opioids are involved in regulating immune defenses. To minimize the increased incidence of infectious diseases in heroin users and individuals clinically exposed to opioids, it will be important to determine the individual and collective effects of the opioid-induced activation of these pathways and the consequences of that activation to the immune system.

Role of central 5-HT(2) receptors in fluoxetine-induced decreases in T lymphocyte activity.

Previous studies have demonstrated that fluoxetine administration decreases mitogen-induced T lymphocyte proliferation. The present studies were carried out to determine which receptor subtype(s) was involved and whether these effects on lymphocyte responses were centrally or peripherally mediated. Two hours following administration of the 5-HT(1A) agonist 8-OH-DPAT (1 mg/kg), there was no change in lymphocyte proliferation responses, whereas the 5-HT(2) agonist DOI (2.5 mg/kg) significantly decreased (80%) proliferation. Similarly, pretreatment with the 5-HT(2) antagonists ritanserin (5 mg/kg, 30 min) or ketanserin (5 mg/kg, 1 h) was found to completely antagonize the effects of fluoxetine on lymphocyte proliferation. Consistent with central 5-HT(2) receptor involvement, microinjection of DOI (50 microg) resulted in a decrease in lymphocyte proliferation similar to that observed following systemic administration. Furthermore, central administration of ketanserin (20 microg) prevented the suppressive effects of systemic fluoxetine. Collectively, these results suggest that decreases in mitogen-induced lymphocyte proliferation following acute fluoxetine administration was due to indirect effects of fluoxetine following the activation of central 5-HT(2) receptors.

Rapid elevation of plasma interleukin-6 by morphine is dependent on autonomic stimulation of adrenal gland.

Several studies have demonstrated that opioids regulate a number of immune cell functions either through direct mechanisms or through the modulation of central nervous system outputs. It has been previously shown that morphine increases serum interleukin-6 (IL-6) levels; however, the mechanism by which this effect is produced is unknown. In the present study, experiments were designed to address the potential role of central opioid receptors, peripheral autonomic ganglia, and activation of the adrenals in the elevation of plasma IL-6 after morphine administration. A rapid and significant (2-fold) increase in plasma IL-6 was observed after morphine administration (10 mg/kg s.c.) to rats. This effect of morphine peaked within 30 min and remained elevated for at least 2 h. Central microinjection of morphine (10 microg/2 microl i.c.v.) mimicked the effects of peripherally administered morphine and was completely blocked by naltrexone (10 mg/kg s.c.) pretreatment. Pretreatment with a ganglionic blocker, chlorisondamine (0.5 mg/kg i.p.), also blocked the elevation of IL-6 by morphine, suggesting a role of the autonomic nervous system. In adrenalectomized animals, morphine administration did not increase IL-6 levels, whereas in adrenal demedullated animals, the effect of morphine remained intact. Thus, the adrenal cortex may be a potential source of IL-6, because IL-6 mRNA has been localized in the adrenal gland. Collectively, these data suggest a unique mechanism by which stimulation of central opioid receptors results in the elevation of plasma IL-6 through autonomic activation specifically of the adrenal cortex.